Of the various electrostatic printing techniques, the most familiar and widely utilized is xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect process since the visible image is first formed on an intermediate photoreceptor and then transferred to a paper surface.
Another method of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). This method differs from the aforementioned xerographic method in that charged toner particles are deposited directly onto an information carrier to form a visible image. In general, this method includes the use of electrostatic fields controlled by addressable electrodes for allowing passage of toner particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the toner particles to an image receiving substrate in image configuration.
A particularly advantageous feature of direct electrostatic printing is its simplicity of simultaneous field imaging and toner transport to produce a visible image on the substrate directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrophotographic printers (e.g., laser printers).
U.S. Pat. No. 5,036,341 granted to Larson discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. According to that method, a control electrode array is positioned between a back electrode and a rotating particle carrier. An image receiving substrate, such as paper, is then positioned between the back electrode and the control electrode array.
An electrostatic field on the back electrode attracts the toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected control electrodes of the control electrode array to create electric fields which permit or restrict transport of toner particles from the particle carrier. In effect, these electric fields open or close selected apertures in the control electrode array to the passage of toner particles by influencing the attractive force from the back electrode to form a modulated stream of charged particles. The charged particles are allowed to pass through selected apertures impinge upon a print receiving substrate interposed in the particle stream to provide line-by-line scan printing to form a visible image.
A drawback of this method is that the charged toner particles which pass through a selected aperture may interact with other electrostatic fields than the intended electrostatic field. This causes toner particles to be deflected from their initial trajectory toward the substrate, and to be displaced from the intended print location thereon. Recent observations indicate that toner deflection can be caused by interaction between transported toner particles and charge accumulations on the substrate side of the control electrode array. In effect, as toner particles are transported through a selected aperture, particles having appropriate charge polarity are attracted by the field from the back electrode and deposited on the substrate to contribute to the formation of a visible image. However, it has been observed that toner may contain a low concentration of particles having a polarity opposite to the intended. Those particles are commonly referred to as wrong sign toner (WST). Since the electric field generated by the back electrode acts to repel WST, the trajectory of WST particles passing through a selected aperture is reversed, whereby WST particles are deposited onto the substrate side of the control electrode array. After sufficient WST particles accumulate on the control electrode array, the electric field produced by them begins to alter the trajectory of right sign toner (RST) toward the print receiving substrate. As a result, transported toner particles are deflected from their initial trajectories due to interaction with accumulated WST charge in the vicinity of the aperture.
A number of different approaches have been proposed to restrict the contamination of the control electrode array. U.S. Pat. No. 4,814,796, granted to Schmidlin, discloses a direct electrostatic printing apparatus including structure for delivering toner particles to a printhead. According to Schmidlin, a DC-biased AC voltage is applied to the toner carrier to excite the toner into a cloud-like state in the neighborhood of the apertures. During printing, the back electrode is electrically biased to a DC potential. Periodically, in the absence of substrate, the back electrode is switched from the DC potential to a biased AC power supply that is 180 .degree. out of phase with the particle carrier AC, such that toner is caused to oscillate and thereby bombard the control electrode array, causing the toner accumulated on the control electrode array to be dislodged. The Schmidlin method requires a periodical cleaning process that must be implemented after one or several pages for removal of dislodged toner particles.
Another attempt to restrict WST accumulation is disclosed in European Patent Application No. 0494454A2. Toner particles are simultaneously stirred and electrically charged within a container by rotating corona elements arranged in a fluidizing bed. Charged toner particles are thereafter electrically attracted onto a first cylinder and transferred to a photoreceptive image cylinder by a plurality of transfer cylinders. A control cylinder is used to remove opposite sign charged toner from an applicator cylinder. The removed toner is vacuumed off the control cylinder. A drawback with this method is that the electric forces applied by the control cylinder may affect the uniformity of the toner layer thickness on the applicator cylinder.
Hence, in order to reduce toner deflection due to WST charge accumulation on the substrate side of the control electrode array, there is still a need to improve a toner delivery system in which toner particles supplied in the vicinity of the printhead have required polarity.